1. Field of the Invention
This invention relates generally to control linkages, and more particularly to control linkages configured to produce synchronous motion in adjacent controllable devices.
2. Background
Conjoined flight control surfaces are often employed on aircraft. One example of conjoined flight control surfaces are dual swept elevator surfaces found at the top of the vertical stabilizer of many “T-tail” aircraft. These dual elevator surfaces are hinged control surfaces adjacently mounted at the trailing edge of the horizontal stabilizer, and act in unison to provide control of the aircraft about the pitch axis. In many T-tail aircraft, dual swept elevator surfaces are actuated by control horns that take the form of swing arms individually connected to adjustable threaded rod ends of a common (bifurcated) pitch control link. During flight maneuvers, torsional forces tend to cause a rocking motion (aeroelastic) of the horizontal stabilizer relative to the vertical stabilizer. This rocking motion tends to induce torque on the bi-furcated pitch control link. This torque will impart a lateral deflection into the control horns and produce differential elevator motion, which can result in a aerodynamically unstable condition, i.e., causing buzz or flutter of the control surfaces.
In many T-tail aircraft, the elevator control horns and bifurcated pitch control link are laterally stabilized by a torque knee linkage attached to the fixed vertical stabilizer in order to address the instability that can result from the twisting rocking motion of the horizontal stabilizer relative to the vertical stabilizer. In such aircraft, the bifurcated pitch control link is nominally stabilized by a forward hinge attachment to a rigidly mounted control lever. Lateral spacing of the individual rod ends provides enough moment couple to allow differential elevator travel and a resultant lateral swing in the pitch control link. The torque knee linkage is attached to the elevator pitch control link to restrain the observed lateral looseness.
Use of the torque knee stabilizing linkage addresses the looseness of the elevator control linkage path in non-flying “static” condition. However, in flight the aeroelastic motion exists between the horizontal and vertical stabilizers tends to induce additional loads and in turn creates a source of wear in the stabilizing system. Experience has shown that wear is induced by the aeroelastic dynamics in the torque knee linkage results in elevator trailing edge freeplay and lateral motion in the control linkages. This wear often necessitates replacement of the torque knee linkage, and requires that a routine maintenance program be implemented to monitor the condition of the torque knee linkage.